It was a jarring corporate end for Irani, who planned to retire from the Los Angeles-based company next year.

The Wall Street Journal said in March that Irani had pressured Chazen to leave but that the CEO had the support of several big investors. The company denied any friction at the top.

"The announcement was basically a formality at this point," said Allen Good, an oil company analyst at Morningstar in Chicago, referring to Irani's departure. "Ultimately this clears the path for Chazen to continue what he is focusing on so far, which is cost improvement. It also might clear the way for the breakup of the company. ... It might make sense to create a couple of different companies out of Oxy."

Occidental said in mid-February that it was seeking a replacement for Chazen. It said in April that the executive would continue in his role through 2014, a move aimed at reducing uncertainty over succession plans.

Earlier this week, influential advisory firm ISS had reaffirmed its recommendation to vote against Irani and for the executive compensation plan, calling recently announced policies a move in a "positive direction."